The present invention relates to a method in the production of a printing ink-decorated packaging material of laminate type, comprising an outer coating of polyethylene which serves as the dxc3xa9cor carrier and which is preferably low density polyethylene (LDPE) having, on its outside applied dxc3xa9cor of water-based printing ink. In particular, the present invention relates to a method in the production of such packaging material for aseptic packages of the type which is produced from a web of packaging material which, for purposes of sterilization, is led through a bath of hydrogen peroxide prior to the reforming of the web into finished, aseptic packages.
Packaging materials of the type described above are known in the art and are often employed in the production of dimensionally stable, liquid-tight packages for liquid foods.
Such a prior art packaging material comprises a core layer of rigid, but foldable paper and outer, liquid-tight coatings of polyethylene, preferably low density polyethylene (LDPE) of which coatings the one has, on its outside, applied decorative artwork (hereinafter referred to as dxc3xa9cor) of water-based printing ink. A similar prior art packaging material for aseptic packages differs from the former substantially only in that its also has a layer serving as gas barrier and disposed between the core layer and one of the two outer polyethylene coatings which is intended to be turned to face inwards when the packaging material is reformed into packages. The material in the layer serving as gas barrier may be a so-called barrier polymer, for example polyamide, polyester, or ethylene vinyl alcohol copolymer (EVOH), but most generally consists of an aluminium foil (so-called Alifoil).
Packaging material of the described, prior art type is conventionally produced in that a web of rigid, but foldable paper is coated with polyethylene, preferably low density polyethylene (LDPE) which is extruded on both sides of the web for the formation of outer, liquid-tight coatings. The plastic-coated paper web is thereafter provided with the desired dxc3xa9cor which, by means of conventional printing technique, is applied on the outside of one of the two outer polyethylene coatings, i.e. that coating which is intended to face outwards when the packaging material is to be reformed into packages.
A packaging material of the prior art type for aseptic packages is produced fundamentally in the same manner as the former packaging material, with the exception that the plastic-coated, undecorated web is provided with a barrier layer, for example an Alifoil, on one side of the web before the web is printed with printing ink.
Nowadays, dimensionally stable, liquid-tight packages are most generally produced from such packaging material with the aid of modem, high-speed packing and filling machines which, either from a web or from prefabricated sheet blanks of the packaging material, form, fill and seal the packages. From, for example a web, the packages are produced in that the web is first reformed into a tube by both longitudinal edges of the web being united with one another in an overlap joint seal. The tube is filled with the relevant contents, for example liquid food, and is divided into closed, filled packages by transverse seals transversely of the longitudinal axis of the tube and below the level of the contents in the tube. The packages are separated from one another by incisions or cuts in the transverse sealing zones and are given the desired, normally parallelepipedic configuration, by a further forming and sealing operation for the formation of the finished packages.
Aseptic packages are produced fundamentally in the same manner as above, with the crucial difference that the packaging material and the contents are sterilized prior to the filling phase, and that the filling phase takes place in a sterile environment so as to avoid reinfection of the sterilized contents.
The sterilization of the packaging material often takes place with the aid of a chemical sterilizing agent, normally an aqueous solution of hydrogen peroxide, with which that part of the packaging material which lies in interface with the sterilized contents is brought into contact. However, the entire material is often sterilized in that the web of packaging material, prior to reforming into a tube as described above, is led down into and through a bath of the aqueous hydrogen peroxide which, for a certain period of time, is allowed to act on the web for eliminating unwanted, in particular pathogenic micro-organisms.
Printing inks for the described application in connection with packaging materials are often water-based and therefore require that the surface on which they are to be applied is sufficiently hydrophilic to achieve good adhesion between the water-based printing ink and the printing surface.
A surface of polyethylene, e.g. LDPE, which is used as printing surface in the prior art packaging material is, however, hydrophobic (water repellent) and must therefore be modified in order that the water-based printing ink be absorbed and adhere with good adhesion to the surface. For the purposes of promoting adhesion, the packaging material is therefore subjected according to the prior art technique to a surface treatment which changes polarity, normally a corona treatment, as a result of which the polyethylene surface obtains the desired hydrophilic character.
While it has previously been possible to produce a printing ink-decorated packaging material of the type described by way of introduction with good adhesion between the water-based printing ink and the outer polyethylene coating, it has surprisingly proved that such a packaging material, in particular for aseptic packages, may be improved considerably in a simple manner and using simple means.
One object of the present invention is, therefore, to indicate how such an improved packaging material, in particular for aseptic packages, may be produced.
This object is attained according to the present invention by the method as set forth in claim 1. Further advantageous characterizing n features of the present invention are apparent from the appended claims.
By replacing the previously employed corona treatment with so-called flame treating, it is possible according to the present invention to produce a printing ink-decorated packaging material which can not only be stored in a damp environment for long storage times without the dxc3xa9cor losing its clarity and freshness, but which also may be sterilized with aqueous hydrogen peroxide, as described previously, with considerably lower consumption of hydrogen peroxide than the corona-treated printing ink-decorated packaging material.
The explanation for the unexpected excellent results attained with the method according to the present invention is intimately related to the surface treatment carried out for the purpose of promoting adhesion. Even if a corona treatment is sufficiently effective to realise the desired modification (change of polarity) of a polyethylene coating, it is at the same time so powerful that the polyethylene coating is xe2x80x9cbroken upxe2x80x9d when it is hit by the electric corona discharges. Such xe2x80x9cbroken upxe2x80x9d regions wholly or partly lack hydrophilic seats which are required for the water-based printing ink to adhere with good adhesion, as a result of which the printing ink thus only partly adheres within these regions. The risk of interaction between printing ink and moisture or liquid with which the printing ink-decorated packaging material may come into contact during storage has thus increased within these xe2x80x9cbroken upxe2x80x9d regions, with resultant action on the applied printing ink which impairs or destroys its quality.
The problem with broken up material regions is particularly serious when the packaging material is, for purposes of sterilization, led through a bath of aqueous hydrogen peroxide solution, as in the production of aseptic packages. Apart from the fact that the printing ink comes into contact and interacts with the water in the bath, the hydrogen peroxide will also be attracted and absorbed by hydrophilic seats in the polyethylene coating in broken up regions outside the applied printing ink dxc3xa9cor, with excessive hydrogen peroxide consumption as a result.